The present invention relates to a control method and equipment for charging ready-mixed concrete additives batchwise, where the charging additives for adjusting the slump are applied to ready-mixed concrete in batches of stipulated size at stipulated times, thus keeping its slump in a certain range without degrading the quality and making longer transportation possible.
When ready-mixed concrete is transported by a truck mixer after being charged at a mixing plant, its reaction proceeds with the progress of time, making it gradually more sticky, harder and less fluid and thus degrading its workability. Generally, the fluidity of ready-mixed concrete is measured as the slump value. Soft ready-mixed concrete of about 20 cm slump is used for steel-reinforced concrete work and hard concrete of about 5 cm slump is used for civil engineering work.
The maximum transportation time of ready-mixed concrete with a truck mixer is normally about one hour and a half, although it may be extended to about 2 hours if ready-mixed concrete of higher slump with added water is charged into the truck mixer, taking a decrease in the slump (hereinafter referred to as slump loss) during transportation into account.
High strength concrete having a water-cement ratio of about 30%, however, shows particularly significant slump loss with the progress of time. Nevertheless, if water is added in excess beforehand to compensate, it will cause the problem that the desired strength of the concrete cannot be achieved. For this reason, it may be said that concrete of this kind should not be transported by a truck mixer but should be mixed at the work site. Still, in view of the quality control or construction setup at the site, the problem should preferably be solved by using ready-mixed concrete transported by truck mixer.
Thus, as a countermeasure against the slump loss of concrete during transportation by truck mixer, additives having properties that may control freely the slump of concrete of this kind during transportation by truck mixer have been sought and developed very recently. These additives are surfactants of a certain type effective not only in preventing slump loss without adding water in excess and without producing any adverse effects on the strength as they are added little by little, while being stirred, into ready-mixed concrete during transportation by truck but also to obtain high strength concrete having excellent quality and workability. The long distance transportation of high strength ready-mixed concrete is thus perhaps possible, although it has been considered so far impracticable.
Still, there remains the problem of how to adjust the dose and interval of charging these additives. The reason is that, if they are added in excess, the ready-mixed concrete will have too high a slump to maintain its workability whereas, if their addition is insufficient, the slump will not be reduced properly. Furthermore, the adjustment may vary depending on the cement quality according to the cement maker, the ambient temperature or the mixing. Unless adjustment is done properly, taking these factors into account, ready-mixed concrete of constant slump may not be obtained.
In this case, although the size of each batch of additives may be determined readily from the amount of ready-mixed concrete, the interval of charging may hardly be determined beforehand as the reactivity of ready-mixed concrete may vary. Nevertheless, from the point of view of safety, the drivers of truck mixers should not have to adjust the additive charge while checking the slump condition of ready-mixed concrete during transportation. This also raises a problem in relation to the quality of ready-mixed concrete.